Abstract:Unless corrected, inaccuracies and noise in images can introduce artifacts in computational imaging, such as localization microscopy, especially when statistical estimation methods such as maximum likelihood estimation are used. The architecture of “scientific” CMOS (sCMOS) camera differs from CCD and EMCCD cameras, and it often believed that photoresponse non-uniformity (PRNU) is relatively high in CMOS cameras. We show through careful measurements of production cameras that photoresponse is highly uniform across the entire array at all light levels, while pixel variance is non-uniform and should be considered on an individual pixel basis. Furthermore, pixel variance is fundamentally an inaccurate method to “calibrate” pixel response, and so doing makes cameras worse than factory calibrations.

In CMOS cameras, the charge to voltage conversion is separate for each pixel and each column has independent amplifiers and analog to digital converters, and pixel-to-pixel variation in quantum efficiency is possible. The “raw” output from the CMOS image sensor, includes pixel-to-pixel variability in the read noise, quantum efficiency, gain, offset and dark current; therefore scientific camera manufacturers digitally calibrate and correct the raw signal from the CMOS image sensors.

To determine the individual pixel output accuracy for production Hamamatsu ORCA Flash 4.0 V3 sCMOS cameras, we measure the distributions and spatial maps of dark offset, and photoresponse uniformity, and overall linearity. Individual pixel variance is modelled as the sum of two terms: dark variance (read noise) plus a term proportional to the signal. Measurements are taken with highly uniform and controlled illumination over low light conditions from dark conditions and at multiple light levels between ~20 to ~30,000 photons / pixel per frame.